Prior resonant converter adopts a voltage-controlled oscillator. A compensation signal is generated from an output voltage of the converter by way of an operational amplifier (or an error amplifier) to regulate the voltage-controlled oscillator, so as to control the switching frequency of the resonant converter. FIG. 1 shows the gain of the resonant converter versus the switching frequency. Typically, the resonant converter operates at the right half region of the critical frequency f0. Thus, the higher the switching frequency is, the lower the gain is.
However, resonant converters with above voltage-controlled oscillation control scheme have poor transient response. Moreover, pulse skipping mode (PSM) is used when the converter is in light load condition in order to improve the efficiency. But the transition between PSM and fixed frequency mode is not smooth, which affects the efficiency and further weakens the transient response.